Integrated circuits (ICs) are typically produced en mass upon single, circular semiconductor wafers having diameters of up to about 30 centimeters (cm).
The semiconductor wafers from which the ICs are cut may have multiple layers of wiring devices on a single wafer. Each layer of circuitry consists of thousands of electrical circuits that will eventually be die cut from the wafer. The successive layers are separated from one another by intervening dielectric layers made of materials such as silicon dioxide. The dielectric and/or metal forming each layers has to be polished or ‘planarized’ before the next layer of circuitry can be deposited. The polishing (or planarization) process is called CMP, which stands for Chemical Mechanical Planarization.
CMP is superior to previously used planarization technologies because it has proven capable of both local and global planarization of the materials used in the fabrication of multi-level ICs. During CMP, a slurry of fine abrasive particles suspended in liquid chemical solutions react with the surface being polished to achieve the necessary degree of flatness prior to the deposition of the next layer.
A layer of insulating material, commonly silicon dioxide or variations thereof, is used to separate each successive layer of the fabricated circuitry so that each sequentially deposited IC layer will not, unintentionally, interconnect with subsequent layers of circuitry. In order to pack more devices into less space, the requirements for feature size within the ICs has shrunk dramatically. Features that protrude into or across circuitry layers and make contact where not intended, or do not make contact where intended, can cause short circuits or open circuits and other defects that make an otherwise valuable product unusable.
One difficulty with CMP is a reduction in the rate at which the CMP pad, or CMP polishing pad, removes material from the wafer being polished and thus the speed of planarization decreases with use. Most conventional polishing pads are made of various kinds of filled or unfilled thermoplastics such as polyurethane. The polishing surface of the pads tends to become glazed and worn during the polishing of multiple wafers. The pad's surface characteristics change sufficiently to cause the polishing performance to deteriorate.
Deterioration of polishing pad performance is typically reversed by the use of means to ‘condition’ the pad surface during use, or between polishing steps, as needed.
The pad conditioning procedure uses a conditioning disk that has diamonds or other hard abrasive particles bonded to it. When this disk is applied to the polishing pad it mills away the top surface of the pad exposing fresh asperities and recreating the micro texture in the surface. Conditioning of the pad is also necessary because the surface of the polishing pads undergoes plastic deformation during use, due to pressure and heat.
Pad conditioning provide a consistent pad polishing performance by periodically regenerating the surface of the pad. Some polishing operations use continuous pad conditioning, others intermittent, some between wafers. The conditioning apparatus generally consists of an arm to which is attached a rotating disk to which is attached the abrasive conditioning surface that rotates while it radially traverses the surface of the rotating polishing pad. The conditioning disk generally has fine diamond grit bonded to its active surface.
Like the pad, the conditioning disk also undergoes wear of its abrasive surface, requiring that it be replaced periodically in a process that requires stopping of the CMP processing of wafer and a consequent reduction in productivity. Thus the conditioning of polishing pads places service-life constraints upon the conditioning disk. A way to increase the operational of the service life of the conditioning disk is thus a desirable goal.
It is worth noting that the rotating conditioning disk also radially traverses the polishing pad while renewing the pad surface and restoring polishing pad performance.
When the conditioning disks are new, the diamond particles are very sharp and quickly ‘roughen’ up the polishing pads. Over time, however, the conditioning effectiveness of the disks decreases until it has to be replaced.